aknflyer, make sure on your setup tab you have enough channels for the mega. You can't just draw the tree and have it display. You also have to have enough universe/channels for the tree. Also on layout tab, you have defined a start channel for the tree.
Most likely, the arch is on universe 1 channel 1 so it plays with no issues.
Most importantly, please have a looksee at the xlights manual. It has a great getting started tutorial in it. There are also several posted on the videos link of the xlights.org site. It is one of the best manuals I have ever seen for a software of this nature. It is a little dated, covers up to version 45, hoping at some point it will be updated again. I had heard at this time it is not being worked on unfortunately, however version available now will get you a long ways.
Finally, Gil's layout in 10 minutes is very helpful as well. This is a great community, feel free to ask questions. Most are friendly in responses, some can be a little terse at times. Good info non the less.
Hope this give you some troubleshooting to get the tree going.
Best,
David